RCD problems

Discussion in 'Electricians' Talk' started by Tangoman, Oct 26, 2019.

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  1. Tangoman

    Tangoman Well-Known Member

    My RCD tripped last week while I was taking a shower.
    I turned off all the breakers and flipped it back on again, however, as soon as I switched the kitchen or the shower breaker back on, the RCD tripped again.

    10 mins of investigation revealed that if ANY circuit had more than minimal load on it, the RCD tripped - it wasn't any particular circuit or appliance that was to blame. Also the Test button on the RCD did not operate.

    So I ordered a replacement from ebay, a used item from a reputable electrical supplier.
    Swapped them round and turned back on again, however as soon as I switched the shower circuit back on, the RCD tripped again - the shower was still on from last time, so it immediately demanded load.
    I now had the SAME issue as before - the RCD tripped if ANY circuit had load and the test button didn't work.

    In order to isolate potential issues, I need to disconnect each circuit on the RCD side of the bar and test them individually right? So a N/E fault on the shower would trip the RCD even if the shower circuit breaker was off unless the circuit was completely disconnected?
     
    Last edited: Oct 26, 2019
  2. Bogle Crag

    Bogle Crag Screwfix Select

    Might be worth a look inside the shower casing and /or any isolator for signs of damage
     
  3. Comlec

    Comlec Screwfix Select

    If you isolate the shower via the local isolator does the RCD stay on?
     
  4. Tangoman

    Tangoman Well-Known Member

    There are 5 circuits on the RCD side
    Shower
    Cooker
    Upstairs Sockets
    Kitchen Sockets
    Boiler

    Forget the boiler circuit for now since it's very low power - assume the isolator is permanently OFF.

    If just ONE of the circuits is ON with ANY appliance on that circuit drawing significant power (I haven't determined the threshold, but cooker, dishwasher, washing machine, PC, shower etc) then the RCD trips again.

    I thought it was the RCD, but then realised that even with the isolators OFF, the neutral is still connected. I think in order to test properly I'll need to disconnect the circuits from the bar, not just turn off the isolators.
     
  5. seneca

    seneca Screwfix Select

    Most "isolator" switches are double pole.
     
  6. Tangoman

    Tangoman Well-Known Member

    OK I hadn't realised that - what do you suggest then?

    The behaviour of the original RCD and the replacement are identical - it's possible, but unlikely that the replacement has the same fault.
     
  7. Comlec

    Comlec Screwfix Select

    I am dealing with a similar problem for a customer. RCD trips when lights are on. Four circuits on RCD.
    Ramp and x5 test on RCD all fine so it is back in daylight to find the culprit.
    And that is why I fit all RCBO boards.
     
  8. peter palmer

    peter palmer Screwfix Select

    Its a rookie mistake not to disconnect the neutrals of each circuit when testing. If the upstairs sockets for eg have a N-E faul then the RCD will trip if the shower is used even if the upstairs sockets MCB is off.
     
    Tangoman likes this.
  9. spinlondon

    spinlondon Screwfix Select

    Where a single RCD protects multiple circuits, a N-E fault on any of those circuits will cause the RCD to trip, if one or more of those circuits is energised.
    If the N-E fault is downstream of a double pole isolator, operating the isolator will prevent the N-E fault affecting the RCD.
    Single pole isolators will not, unless they are wired with reverse polarity.
     
    Peterdevon and Tangoman like this.
  10. Tangoman

    Tangoman Well-Known Member

    Well disconnecting the shower neutral works - at least the RCD no longer trips, but I have another issue I need to investigate when I get chance.
    The shower neutral had actually melted into the boiler neutral (which is not on the RCD), which I think means that the boiler could be responsible for the issue since effectively the neutral from the boiler was connected (via the shower cable) to the RCD side, and both RCD and non-RCD circuits share earth.
     
  11. peter palmer

    peter palmer Screwfix Select

    Kind of, with the fault you describe the 2 neutral bars are effectively joined together so a portion of the current that is supposed to flow through the RCD from all or any of the RCD protected circuits would flow via the non RCD neutral bar and cause an imbalance of current in the RCD causing it to trip.

    But equally the current from non RCD loads like the boiler can now flow through the RCD neutral bar and ultimately through the RCD, this also would cause an imbalance but in the opposite direction to normal faults, so its possible switching on something on the non RCD side of the board could still trip the RCD.

    The fault you describe though is undoubtedly caused by a loose or bad connection, more than likely on the shower neutral wire in the bar. Any significant load on that wire and it will quickly get very hot melting the insulation off it. Its not uncommon for this sort of fault to end up destroying the whole consumer unit and if plastic ultimately setting fire to the house, the RCD did its job and you caught the potential disaster early.
     

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